Electrically switchable photonic crystals are demonstrated based on TiO2 inverse opals infiltrated with liquid crystals. Macroporous anatase TiO2 inverse opals are fabricated from polystyrene opal templates through a sandwich vacuum backfilled method and followed by calcination. Upon liquid crystal infiltration, the optical properties of the hybrid organic/inorganic structure are characterized by reflectance measurements of the Bragg peak, the position of which can be switched using an external electric field. The physical mechanism underlying this switchable behavior is the reorientation of the liquid crystal molecules inside the spherical voids by the applied electric field, resulting in a significant change of the refractive index contrast between the liquid crystal and the TiO2 inverse opal. With advantageous features of cost-effective fabrication, easy integration, and electric control, such TiO2 inverse opals infiltrated with liquid crystals could play an important role in future development of active photonic devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Photonic crystals (PhCs) that enable the molding of the flow of photons , hold the great promise for an emerging generation of micro-/nano-scale optoelectronic components. Opals and inverse opals (IOs) are a kind of three-dimensionally (3D) ordered PhCs [2,3], which are prepared by the self-assembly of monodispersed colloidal particles and their derivatives. Upon the light incidence, interferences between strongly scattered waves lead to a modification of the photon density of states in particular wavelength regions . In recent years, they have attracted growing interest for many potential applications including optoelectronic devices (optical switches or lasers) [5–7], sensing [8,9], protein patterning , and so on. The performance of these devices is mainly related to the lattice periodicity, the quality and the size of the ordered structure and the refractive index contrast.
Recently, titanium dioxide inverse opals (TiO2–IO) PhCs have gained particular attention because of its excellent optical and electronic properties [11–15]. Based on the slow photon effect, TiO2–IO PhCs have demonstrated greatly enhanced photocatalytic and photovoltaic activity [16–19]. Especially, anatase TiO2 has shown excellent potential on the treatment of industrial wastewater containing harmful aromatic compounds and organic dyes [20–22].
However, many potential applications of PhCs require active control over the photonic bandgap (PBG) structure through external stimuli. The PBG tuning is mainly related with the lattice constants and the refractive index. In recent years, it has been reported that various approaches have been exploited to tune the PBG, including the volume phase transition of hydrogels by controlling the temperature or the pH [23,24], electrically [25–27] and optically [28–32] controlled birefringence of liquid crystal, etc. However, the low index contrast in the LC-infiltrated SiO2-IO material system greatly limits the optical performance. Instead, high index contrast in the LC-infiltrated PhCs is highly desirable to enhance its optical performance.
In this study, we report an electrically switchable PhC based on LC-infiltrated TiO2-IOs. The TiO2-IOs was fabricated via a sandwich-vacuum technique by utilizing self-assembled polystyrene (PS) colloidal spheres as the template . Upon LC infiltration, the large voids inside the TiO2-IOs provide the low anchoring energy for the LC molecules due to the relatively low surface-to-volume ratio, hence facilitating the easy control of the LC molecules’ alignment. Driven by an external electric field, the LC molecules can be realigned, showing a large change of the refractive index and hence leading to switchable Bragg reflection. With facile and cost-effective fabrication, such an electrically switchable PhC holds great potential for many applications including structural colors, optical filters, etc.
All reagents and solvents were purchased from commercial suppliers and used without further purification unless specified. Styrene (St, analytically pure) and potassium persulfate (KPS, analytical pure) were purchased from Energy Chemical Reagent Co. Deionized water (> 18.2 MΩ cm) was used in all polymerization processes of this study. Titanium (IV) butoxide (97%, Sigma-Aldrich) was purchased from Aladdin.
2.2 Synthesis of PS microspheres
An emulsifier-free emulsion polymerization approach was used to synthesize monodisperse PS colloidal microspheres . The polymerization was carried out in a four-necked jacked glass reactor equipped with nitrogen bubbler. Deionized (DI) water (100 mL) was firstly introduced into the glass reactor purged with nitrogen for 10 min. Then, styrene was added into the reactor, and mixture was stirred vigorously at 330 rpm for another 10 min below 70°C. A small amount of KPS aqueous solution was added, and the polymerization reaction was carried out at 70°C for 20 h with nitrogen protection. After that, the mixture was cooled down to room temperature and filtered to remove coagulum. The filtrate was kept for the following experiment.
2.3 Fabrication of PS opal films
All substrates (FTO) were subsequently cleaned with acetone, ethanol and DI water in an ultrasonic bath for 15 min, and then treated with O2 plasma for 15 sec. Then the FTO glass substrates were washed with copious DI water and dried under nitrogen gas prior to use . PS colloidal spheres with the size of 200 nm and 250 nm in diameter were synthesized for the present study. PS colloidal crystals (opal) films were self-assembled on FTO substrates via a vertical deposition method .
2.4 Fabrication of TiO2 inverse opals
10 mL of titanium (IV) butoxide, 10 mL anhydrous ethanol and 1 mL nitric acid were vigorously stirred for 30 minutes to obtain homogenous titanium (IV) butoxide sol. Before infiltration with the TiO2 precursor solution, the PS opal films were annealed at 80 °C for 20 min. A sandwich-vacuum technique was employed for the infiltration (Fig. 1) . Typically, the PS opal films template was covered with a smooth glass slide and pressed together with two clips. The close contact between the opal template and the glass slide may avoid the formation of a TiO2 overlayer. Then half of the template was immersed in the modified TiO2 precursor solution, and the other half was exposed to air. The container holding the TiO2 precursor solution was sealed with a stopper and connected to a vacuum pump. After infiltration, the opal/TiO2 composite was allowed to dry for 24 h in air. Then the samples were calcined at 450 °C with a ramp rate of 1°C/min and kept at 450 °C for 4 h. After selectively removing the PS opals via calcination, anatase TiO2-IO with a well-ordered open macroporous structure were obtained .
2.5 LC infiltration
Once the TiO2-IO films were ready, we assembled them with the bare ITO glass substrates to form a LC cell. The thickness was controlled to be ~20 μm using the PS microspheres. The LC E7 (Merck) material was then injected to the empty cell inside a vacuum chamber. Under vacuum condition, the interstitial spaces of the TiO2-IO films were easily infiltrated with the LCs due to the external pressure and capillary force.
The morphology, size distribution and composition of as-prepared nanocrystals were investigated by the field emission scanning electron microscopy (FESEM, Merlin, Zeiss) with an energy-dispersive X-ray spectrometer (EDS). The compositions of films were characterized by X-ray diffraction (XRD) using a Bruker by X-ray diffraction (XRD) in a D8 advance X-ray diffractometer (Bruker, Switzerland) at a scanning rate of 0.2 °/min with a Cu Kα radiation (151.54056A˚). Optical reflection spectra were measured with an unpolarized probe light beam using a UV-Vis-NIR microspectrophotometer (20/30 PVTM, CRAIC). The broadband light beam was focused to have a probing area of 10 × 10 µm2 using a 10 × objective lens combined with a variable aperture.
3. Results and discussion
Figure 2 shows FESEM images of the PS colloidal crystal fabricated by using a vertical self-assembly method . A well-ordered 3D structure was clearly revealed from FESEM images with PS opals self-assembled with both 200 and 250 nm PS microspheres in diameter [Figs. 2(a) and 2(b)]. Figures 2(c) and 2(d) shows the resultant TiO2-IOs after calcination with the estimated sizes in diameter of the spheroidal voids being ~180 nm and ~207 nm, respectively. Comparing to the corresponding PS spheres used in our experiments, there was a substantial decrease in diameter for the size of the spheroidal voids due to the shrinkage during the calcination of PS spheres. It should be noted the shrinkage ratio varies a little for these two different sizes of PS spheres.
Figure 3(a) shows the XRD spectra of TiO2-IOs sample and typical characteristic peaks from anatase TiO2 nanocrystals can be observed. The strong diffraction peaks located at 2θ values of 25.3°, 37.8°, 48.1°, 53.9°, and 55.1°, which can be assigned to (101), (004), (200), (105), and (211) reflections of anatase TiO2 nanocrystals, respectively . Figure 3(b) shows the EDS spectra of the TiO2-IOs sample, which confirms that Ti and O are the major elements. In addition, there exist signals for elemental Si, Ca, Na, and Pt in the EDS spectra, which come mainly from the FTO glass substrate, and the thin conducting layer (i.e., Pt) with the purpose for SEM observation.
In our experiments, we prepared the PS opal templates and their corresponding TiO2-IO samples using the PS spheres with two different sizes of 200 nm (PS opals-200) and 250 nm (PS opals-250) in diameter. Figure 4 shows the measured reflection spectra of PS opals and TiO2-IO films, respectively. Distinct Bragg reflection peaks of PS opals-200 and PS opals-250 appear at 531 nm and 607 nm on the reflection spectra, respectively [Fig. 4(a)], indicating a well-ordered structure of the PS opals. Figure 4(b) shows the reflection spectra of the TiO2-IO films. The Bragg reflection peaks of TiO2-IOs from the 200-nm and 250-nm PS opal templates appear at 452 nm and 528 nm, respectively, showing a significant blue-shift. The blue-shift is caused by the decreased effective refractive index and reduced pore sizes.
As previously discussed, the fabricated TiO2-IOs are closely packed face-centered cubic (FCC) PhCs. Under normally incident probe light, the Bragg reflectance results from the first order diffraction from the (111) planes of the IO structures. The reflectance peak for the (111) plane of a FCC lattice can be analytically determined by Bragg’s law, as follows :
By using the Bragg diffraction equation, we calculated the stop bands of both PS opals and TiO2-IOs. For PS opals, according to the FCC crystal lattice, we assume that the volume fraction of PS is 74% while the volume fraction of air is 26%. The λmax of PS opals-200 and PS opals-250 is calculated to be 475 nm and 594 nm, respectively [here it was assumed that the refractive index of PS is 1.59 and the refractive index of air is 1, thus neff = (1.592 × 0.74 + 12 × 0.26)1/2 = 1.460]. While for the corresponding TiO2-IOs, λmax is calculated to be 465 nm and 536 nm, respectively [here it was assumed that the refractive index of TiO2 is 2.61 and the refractive index of air is 1, thus neff = (12 × 0.74 + 2.612 × 0.26)1/2 = 1.585]. We notice that the calculated stop band wavelengths for both PS opals and TiO2-IOs do not match well with the measured ones, which could be mainly attributed to two major possible reasons: 1) the assumed volume ratio varies due to the shrinkage; and 2) the assumed refractive indices of PS and TiO2 should be larger than the actual case.
Compared to both opals structure and inverse opals structure, the structure provides a much less constricting volume for LC reorientation under applied fields, which should facilitate the largest possible optical tuning . The sample was infiltrated with LC (E7) using an oven. Figure 5(a) shows the measured reflection spectra before and after LC infiltration for the TiO2-IOs with a pore size of 180 nm. Before LC infiltration, the TiO2-IOs showed a Bragg reflection peak at 452 nm and exhibited a full-width at half-maximum (FWHM) of 51 nm. Upon LC infiltration, the reflection peak red-shifted from 452 nm to 636 nm with a FWHM of 48 nm. The red-shift of the Bragg reflection peak can be attributed to the increase in the average refractive index of the film resulted from the replacement of air with LCs. Moreover, there is a significant decrease in reflectance due to the decreased refractive index contrast for the LC-infiltrated case, which is in good agreement with the expectation that the Bragg reflection peak collapses when the effective refractive indices between the spheroidal LC droplets and TiO2 completely match each other. It is worth mentioning that there was no alignment layer in our experiments. We assumed that the LC molecules inside the voids were randomly aligned, hence demonstrating an averaged refractive index.
One distinctive feature for the LC-filtrated TiO2-IOs is that the LC molecules can be realigned by an external electric field, making their optical properties switchable/tunable. Upon applying a voltage, the LC molecules inside the spheroidal voids can realign along the electric field direction, hence changing the effective refractive index of the LC. Once the applied voltage is removed, the LC molecules will return back to the original state. As a result, the refractive index contrast between the LCs and TiO2 can be reversibly changed, hence leading to the change of the PBG properties. In our experiments, the reflectance spectra of one sample were measured as a function of applied AC voltage with the frequency of 25 kHz, as shown in Fig. 5(b). With the applied voltage, the Bragg peak continuously blue-shifts. With the increase voltages from 0 to 45 V, the Bragg peak demonstrates a slight blue-shift; more importantly, the reflection peak becomes much flat. We can see that the stop band could be completely switched at the voltage of 45 V, indicating a complete index match between LCs and TiO2. At the driving voltage of 45 V, the measured rising and falling times were 5.5 and 3.0 ms, respectively. This complete switching behavior is different from the previous report , making it potentially useful for filtering application. It is worth noting that after switching off the Bragg peak, there still exists a broad peak with the center wavelength locating at ~550 nm. This broad peak may come from multilayer interference in the LC-filtrated TiO2-IOs.
In conclusion, we have fabricated 3D macroporous anatase TiO2-IOs from the PS opal templates through a sandwich vacuum backfilled method and followed by calcination. By infiltrating the TiO2-IOs with a nematic LC, we have demonstrated electrically switchable PhCs. The LC-infiltrated TiO2-IOs possess a distinctive Bragg reflectance. Upon applying an external electric field, the Bragg reflectance peak can be switched due to the reorientation of the LC molecules inside the spherical voids. With advantageous features of low-cost fabrication, simple integration, and electric control, such LC-infiltrated TiO2-IOs could play an important role in future development of active photonic devices, such as switches , filters [40,41], and structural colors [42,43].
Natural Science Foundation of Guangdong Province (Grant No. 2017A030313034 and 2018A030310224), Shenzhen Science and Technology Innovation Council (Grant No. JCYJ20170817111349280 and KQTD2016030111203005), National Natural Science Foundation of China (Grant No. 61805113), and Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2017ZT07C071), and the joint French-Singaporean MERLION program (Grant No. R279000334133).
The authors thank the Key Laboratory of Municipal Solid Waste Recycling Technology and Management of Shenzhen City, Southern University of Science and Technology (SUSTech) for use of their facilities and equipment.
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